The present invention relates to a method for controlling the pitch of a marine propeller suited for installation on outboard motors and Z-drives, among others. The invention is based on a mechanism that rotates each one of the propeller blades about a radial axis that is essentially orthogonal to the propeller axis and thus implements the adjustment of the blade angle-known as pitch.
For blade pitch adjustment, the most commonly used mechanism type comprises a lever arm connected to the blade root, generally orthogonal to the rotation axis thereof, and a cylinder rod that is connected to the lever arm and is adapted to move axially in respect to the hub bore in a parallel direction to the propeller shaft. The blade is mounted on the hub by means of radial and axial bearings. The movement of the piston rod is typically achieved by means of hydraulic servomotors that may be arranged to operate in the interior of the hub and rotating with the hub, or be of the stationary type and be placed about the hub.
In applications implemented by means of rotary servomotors, a hydraulic fluid reservoir must be provided wherefrom hydraulic oil or other pressurized medium is delivered by a stationary pump to the rotary servomotors. Such a hydraulic fluid reservoir that has seals specified for a high pressure and rapid slide movements for an extended period of time is complicated to manufacture and highly subject to leak. Also its size becomes excessively large for adaptation to, e.g., a normal-size propeller hub of an outboard motor so that a sufficient space can be reserved for exhaust channels and the like. If the pressurized-medium reservoir is located outside the propeller hub, the space needed by such constructions cause disturbance to the water flow about the propeller. An example of such prior-art-construction can be found in U.S. Pat. No. 5,226,844.
The flow of the pressurized medium to the servomotors is conventionally controlled by valves that may be placed exterior to the pressurized-medium reservoir or directly into the hub as is taught in U.S. Pat. No. 4,781,533. The valves themselves are controlled by a separate mechanism such as an auxiliary servomotor. If the valves are placed in the hub, the control mechanism must typically be designed capable of transmitting the movement of a stationary source of control power via, e.g., a slide ring to a rotary valve shaft. Such a mechanism conventionally requires a hollow propeller shaft with substantial space thereabout, which makes it difficult to develop control mechanisms suitable for, e.g., retrofit to an outboard motor. In applications known in the art, the control valves are connected so that the pressurized-medium reservoir is kept continuously pressurized when the propeller is running. Resultingly, as mentioned above, the seals must endure high pressure and rapid sliding movements for long periods of time.
Constructions implemented using stationary servomotors need a transmission with bearings for applying the axial force to the cylinder rods that rotate with the propeller shaft as is taught in U.S. Pat. No. 4,142,835. Herein, high forces are encountered and they require the use of big axial bearings that are obviously difficult to adapt into a small space.
The above-mentioned techniques suffer from frictional power loss occurring in both the sealing assemblies of the pressurized medium reservoir and the axial bearings. Other disadvantages of these prior-art techniques are that the piping between the servomotor and the check valves of the hydraulic pump become long and, hence, the volume of the pressurized medium in the piping is large. This causes play and inaccuracy in pitch control due to expansion of piping and a certain degree of com-pressibility of the pressurized medium, particularly if the medium is not air-free.
In U.S. Pat. No. 3,690,788 is disclosed a propeller mechanism, wherein the flow of pressurized fluid is cut off by means of a spool-like valve between the servomotor controlling the pitch of the propeller blades and the pressurized fluid reservoir. As the spool thus prevents the flow of pressurized hydraulic fluid to the servomotor and away therefrom, the propeller blades are locked to a fixed pitch. The space containing the pressurized fluid reservoir is under pressure only when the propeller pitch is being altered. In the above-described construction, the spool controlling the flow of the pressurized medium is located aft of the end of the propeller shaft coaxially with the propeller shaft, which arrangement makes the propeller hub enclosing the mechanism long and very large in size. Hence, the construction is difficult to adapt to plural applications.
It is an object of the invention to overcome the disadvantages and problems hampering prior-art constructions described above.
The goal of the invention is achieved by cutting off the flow of the pressurized medium between the servomotor controlling the propeller pitch and the space containing the pressurized-medium by means of a pressure-controlled valve assembly when the pressure in the pressurized-medium space falls at a given low level. Thus, the valve assembly prevents inflow of the pressurized medium to the servomotor and outflow therefrom, whereby the locking of the propeller pitch to a fixed pitch is effected. The valve assembly is located into a sleeve enclosing the propeller shaft, advantageously radially, whereby the valve assembly does not occupy any extra space in the axial direction of the propeller shaft.
The invention offers significant benefits.
Accordingly, the invention makes it possible to implement a controllable-pitch propeller featuring reduced wear and friction in the seals of the pressurized-medium reservoir. The hydraulic play is minimized through minimized volume of pressure spaces in the system. The mechanism according to the invention occupies only a small space in the hub thus permitting retrofitting on outboard motors, for example.
In a controllable-pitch propeller according to the invention, the pressurized medium reservoir, wherefrom the pressurized medium delivered thereto by a stationary pump is passed to one or more servomotors rotating with the propeller shaft, is unpressurized at times when pitch control is not effected. In the following, the number of servomotors is assumed to be one, but obviously a greater number may be used depending on a specific application. Locking of piston movement in the servomotor is implemented by means of a valve assembly that with a falling of the pressure in the pressurized-medium reservoir cuts off the flow of the pressurized medium into and out from the hydraulic cylinder compartments at both sides of the piston. In reality, this means that the propeller becomes locked to a fixed pitch without any need to subject the seals of the pressurized-medium reservoir to pressure during the time the pump is nonoperative. A characterizing feature is that the valves rotate with the servomotor about the propeller shaft and that they are connected between the pressurized medium reservoir and the servomotor. Additionally, the valves shall have such a construction that requires no external steering mechanism, whereby the valves are controlled in a self-contained manner by the pressure acting in the pressurized-medium reservoir.